1. Technical Field:
The present disclosure relates to fuel cell systems including a hydrodesulfurizer for removing sulfur components present in raw materials.
2. Description of the Related Art:
Solid oxide fuel cells (SOFC), which are a type of fuel cells operating at high temperatures, have been developed progressively. In solid oxide fuel cell systems involving hydrocarbons as raw materials, the raw materials are reformed by, for example, steam reformation using steam. Steam reforming catalysts are used to promote this steam reformation. These steam reforming catalysts are vulnerable to degradation by odorants or sulfur compounds present in the raw materials. To prevent the steam reforming catalysts from being degraded, desulfurizers are used which decrease the amounts of odorants or sulfur compounds present in the raw materials.
The desulfurizers contain a desulfurizing catalyst, which exhibits varied performances depending on the ambient temperature. It is therefore necessary that the desulfurizer be heated in the case where the desulfurizing catalyst that is used exhibits higher performance in the removal of sulfur components at higher temperatures than at normal temperature. For example, Japanese Unexamined Patent Application Publication No. 2006-309982 proposes a solid oxide fuel cell system which is such that a desulfurizer is heated by the use of a high-temperature exhaust gas produced during the power generation in the solid oxide fuel cell.
To allow a desulfurizer to be operated at an appropriate temperature, for example, Japanese Unexamined Patent Application Publication No. 10-214632 discloses a fuel cell power generation apparatus configured such that the desulfurizer is fitted with a temperature sensor for detecting the inside temperature and with a cooling unit that decreases the temperature of the desulfurizer to a prescribed temperature based on signals from the temperature sensor. In the fuel cell power generation apparatus disclosed in Japanese Unexamined Patent Application Publication No. 10-214632, it is proposed that the cooling unit be such that a cooling fluid is passed through a cooling heat exchanger disposed in a hydrodesulfurizing catalyst layer or through a cooling pipe wound around the outer wall of the desulfurizer. The cooling fluid that is passed through the cooling unit is combustion air to be supplied to a combustor which heats a reformer.
Further, for example, Japanese Unexamined Patent Application Publication No. 2012-155978 proposes a fuel cell system which utilizes part of an oxidizing gas or cooling water as a cooling medium so that the temperature of an exhaust gas for heating the desulfurizer may be decreased to a prescribed temperature range. Specifically, as illustrated in FIG. 9, the fuel cell system described in Japanese Unexamined Patent Application Publication No. 2012-155978 is such that the temperature of an exhaust gas to be introduced into a desulfurizer 106 is decreased with a cooler 107 which utilizes a diverted part of an oxidizing gas as a cooling medium, to a prescribed range of temperatures allowing a desulfurizing catalyst to exhibit the performance appropriately.
However, these conventional fuel cell systems are still unsatisfactory in terms of power generation stability.